The present invention relates to display devices and, more particularly, to an improved backlight system for liquid crystal device ("LCD") displays, designed for use in daytime, and with night vision imaging systems.
1. Review of the Prior Art.
LCD's have been employed as modulators of light, and, as such, have been extensively employed in various devices as displays for information in various forms and formats. Reflective LCD's rely upon impinging illumination which enters the face of the device, is reflected from a rear surface, and exits the face. Reflective LCD's have the advantages of lower power requirements, and reductions in size and weight compared to transmissive LCD's, which must have illumination sources.
Typical uses of reflective LCD's are in wrist watch faces and some lap top computer screens. These require ambient light to provide the contrast between the light transmissive and opaque portions of the display. Reflective LCD's do not work effectively in a dimly lighted environment, and are completely ineffective in darkness. Some watchmakers solve that problem by providing a small light source in the watch to illuminate the face, creating the necessary contrast between light and dark. However, some of the benefit of the reflective device (i.e., lower power and fewer components) is lost thereby.
While suffering in the presence of low ambient illumination, the reflective device is also sensitive to the light transmissive ability of the LCD, since the luminance of the display is dependent upon the square of the transmission of light through the LCD. This is because the ambient light must travel through the LCD to the reflective surface and then back out through the LCD again.
Typically, a monochrome twisted nematic LCD, such as are used in watch faces, can have light transmission of up to 35%. This is adequate for reflective mode viewing, where the luminance level of the display will be about 10% of the viewing ambient.
In contrast, Full Color, active matrix LCD displays must use artificial illumination devices due to the extremely low rate of light transmission through the display. Creating a color display requires the incorporation of absorptive color dye filter mosaics along with complex, integral addressing structures. The absorptive dyes and addressing structures allow light transmissions in the range of less than 5%, and in the reflective mode would have a luminance of approximately 0.25% of the ambient light level, which is unacceptably low.
A typical backlight illumination source is a fluorescent lamp, which has the requisite properties of high efficiency, long life and good color balance (i.e., transmits the full range of frequencies within the visible spectrum in their normal concentrations).
Transmissive color LCD displays have found application in color television projection systems and in "flat screen" color displays for computer systems. Yet another application for such devices is in avionics multi-function displays, which are required to present both full-color graphics and multi-gray scale video imagery. Such a display must have high brightness and contrast for good sunlight viewing, as well as a wide dimming range for night time operation.
Another requirement for avionic displays is compatibility with night vision goggles, which are used to view exterior scenes under totally dark conditions. Such night vision devices employ high gain electronic image intensifiers that are sensitive to light in the near-infrared and infrared spectral regions. To be compatible with such systems, cockpit displays must provide sufficient brightness for direct viewing with the unaided dark adapted eye, while, at the same time not emit energy in that portion of the spectrum that might saturate the night vision equipment or cause a reduction in image intensifier gain. Further, the display should not otherwise adversely effect the ability of such equipment to provide real-world night vision. The quantitative requirements for compatibility with such devices is given in Military Specification MIL-L-85762A, dated Aug. 26, 1988.
While these are seemingly contradictory requirements, they are compatible because the spectral region of maximum night vision equipment sensitivity is not coextensive with the spectral region of maximum human visual sensitivity. Night vision equipment is most sensitive to light in the infrared region (approx. 780 nm) while the human eye is most sensitive to light in the yellow-green region (approx. 550 nm). Accordingly, one could filter the display emissions to minimize the longer wavelength components that would otherwise interfere with the night vision equipment without adversely affecting that portion of the visible spectrum to which the human eye is most sensitive.
In dealing with a cockpit display that uses a backlit LCD, whose emission is mainly in the shorter wavelengths (i.e. green-blue), the back light can be easily filtered to remove the infrared and near infrared components without adversely affecting the display luminance. However, there is a penalty associated with this scheme whenever a multi-color display is involved. That is because the sensitivity of the night vision systems extends into the "red" region for some devices (Class B of MIL-L-85762A) and into the "red-orange" region for more sensitive devices (Class A of MIL-L-85762A). Some night vision systems are so sensitive that display emissions within the range of sensitivity must be strongly attenuated, including those in the visible red and red-orange region of the spectrum.
Attenuation of the longer wavelength portions of the visible spectrum reduces the brightness of the colors "red" and "red-orange", and can distort the display color balance. However, the color balance can be partially restored by adjusting the primary color emission ratios in the backlight lamps. The net result is a reduction in luminous efficiency, a lowering of peak display brightness and high ambient contrast, and a degradation of sunlight readability.
Driving the backlight lamps "harder" to achieve higher brightness levels for the non night vision applications will partially ameliorate the problem, but at the cost of higher overall power dissipation, increased weight and shorter lamp life.
With a monochrome LCD display, prior art approaches have utilized a light source with an infrared attenuating filter to achieve compatibility with night vision equipment. One such system is disclosed in the recent patent to Klein, U.S. Pat. No. 4,934,793. Such a system is subject to all of the shortcomings outlined above, should a full color system be employed.
Sanai et al., U.S. Pat No. 5,029,045, Horiuchi, U.S. Pat. No. 4,998,804, and Suzawa, U.S. Pat. No. 4,618,216 teach the full-color illumination of LCD's, while Tanaka et al., U.S. Pat. No. 5,040,098, allows illumination of monochromatic LCD displays whose color attribute is adjusted by means of organic fluorescent dyes. The LCD illumination device of Hamada, U.S. Pat. No. 4,914,553 utilizes a Fresnel reflection surface, and can be used with either built-in or external light sources. Masuzawa et al., U.S. Pat. No. 4,642,736, teaches a light diffuser for normal illumination of various types of machinery as well as LCD devices. None of these references are intended for use with modern night vision goggles.
Other prior art devices for illuminating conventional, non-LCD panel instruments to be useful with night vision goggles include Garret, U.S. Pat. No. 4,887,189, and Reiter et al., U.S. Pat. No. 4,581,683. The device taught in Garret uses a "hot", incandescent light bulb that requires filters to remove unwanted wavelengths of light in the red and infrared band, and teaches a method of cooling the bulb to prevent premature burnout. The device in Reiter is specifically designed to emit only green light, and, accordingly, each is even less applicable for use with full color LCD's.
The primary object of the present invention is to permit the reading of an LCD display at night with or without the use of night vision goggles, and with no diminution of system efficiency or readability during the course of daylight viewing.